1. Field of the Invention
The present invention relates to a sound pitch converting apparatus such as a KARAOKE (sing along a melody) player and a sound and image editor for shifting sound pitch or an original frequency of the sound and particularly to an apparatus which can easily shift a sound pitch maintaining the original sound characteristics without causing sound deterioration.
2. Description of the Related Art
A conventional sound pitch converting apparatus such as a conventional karaoke player has a function called a key control for shifting a pitch of accompanying sound to adjust it to a singing player's register. This key control shifts the musical sound pitch by changing a reproducing speed of the accompanying sound of analogue signal.
Recently, a communication karaoke system has been developed, in which a music provider stores a wide variety of songs and delivers them to a plurality of terminal users in response to their requests.
Digital data of such a delivered song consist of character data for displaying and changing colors of characters synchronously with an accompaniment music, a MIDI (Musical Instrument Digital Interface) signal for driving terminal synthesizer to reproduce the accompaniment music, and a compressed sound signal for reproducing natural voices of male or female accompaniment chorus.
The MIDI signal of this karaoke system can be changed in their sound pitches by controlling settings of the synthesizer to be higher or lower in frequency than the original pitch, without changing the original tempo.
However, it is difficult to change the sound pitch of the natural voices of male or female accompaniment chorus without alterations of its tempo and characteristics of the original voices, and without causing deterioration of the sound quality, because it is not a MIDI signal but an analogue signal without having a pitch control information.
Recently, an audio/video editing apparatus is developed which edits digital sound signals, however, it fails to change sound pitches without losing high quality of original sounds.
There are mainly two conventional methods which change sound pitch but keep an original tempo.
One of them is a method of sampling and processing a sound signal in a time domain. When the sound pitch is intended to be raised two times the original for example, the sound signal is divided into predetermined segments, and data of these divided sound signals are read out at two times of the original readout speed to obtain a doubled pitch signal. Or, a pitch frequency (the lowest frequency exhibited when a divided signal segment is analyzed in its frequency spectrum, "pitch frequency" is also called "fundamental frequency") of each of the divided sound signal segments is detected and doubled to obtain the doubled pitch signal. In either case, a divided time period corresponding to the predetermined segment is filled up by using the doubled pitch signal repeatedly. Thus, the pitch frequency is doubled without changing the original tempo of the sound. A problem in this method is smooth connection of the doubled pitch signal segments. In fact, the reproduced sound is deteriorated because of an imperfect connection, and the characteristics of the original sound is distorted.
Another method uses a Fourier transform which deals with the sound signals in a frequency domain. The sound signal is divided into a plurality of predetermined segments.
Amplitude and phase components of the divided signal segments in the frequency domain are extracted by a Fourier transform, and are shifted by desired amounts respectively.
Then, the shifted amplitude and phase components are reformed back to the time domain by inverse-Fourier transform. After that, the pitch changed sound signal segments are connected each other. However, this method has been evaluated by the present inventors that the reproduced sound is unnatural and unacceptable.
Japanese patent Laid-Open Application No. 59-204096/1984 by the present applicant discloses another method using a Fourier transform. The sound signal is divided into a plurality of predetermined segments, which are then transformed by Fourier transform. A pitch frequency of the transformed sound signals is detected. Only components around this detected pitch frequency are shifted by a predetermined value.
The method disclosed in Japanese patent Laid-Open Application No. 59-204096/1984 has a problem that harmonic sounds left without shifting remind a listener of their original pitch. As a result, the listener hears both of the original and the shifted pitch sounds.
There is a similar pitch change requirement in other systems, such as tape recorders or VCRs, than the KARAOKE players, in those tape recorders or VCRs, the original sound pitch is desired to be kept when such apparatuses play in higher speed than the standard one.